1. Field of the Invention
The present invention relates to an ultrasonic diagnostic apparatus for obtaining blood data by use of Doppler effect.
2. Discussion of the Background
FIG. 1 shows the structure of a conventional ultrasonic diagnostic apparatus corresponding to a spectrum Doppler mode. In this figure, a pulse generator 121 generates a rate pulse at a period of a reciprocal number of a pulse repetition frequency (PRF). A pulser 103 generates a voltage pulse having a high frequency in synchronous with the rate pulse. A piezoelectric vibrator of a probe 101 is vibrated by the voltage pulse. Thereby, an ultrasonic pulse is transmitted to a subject. A central frequency (transmission frequency) of the ultrasonic pulse is expressed by c.
The ultrasonic pulse is reflected at a boundary of acoustic impedance of the subject, and the part of the reflected ultrasonic wave is returned to the probe 101. Though the ultrasound is weak, the ultrasound scatters even in blood corpuscles. Since the blood corpuscles are moved, the frequency of the ultrasound is shifted in accordance with the velocity of the corpuscles. The spectrum Doppler mode observes the shifted frequency fd. The frequency fd can be obtained by the following equation: EQU fd=(2.multidot.V.multidot.fc.multidot.cos .theta.)/C
wherein a blood velocity is V, an angle between an ultrasonic beam and a direction of a blood current is .theta., and a sound velocity of a living body (about 1530 m/sec) is C. The center frequency of the ultrasound is fc.
To obtain the shift frequency fd of the blood current, an echo signal is amplified by a preamplifier 105 and orthogonally detected through a mixer 107 and a low pass filter 109. Thereby, a Doppler signal corresponding to a shift frequency component can be obtained.
Then, the Doppler signal from the depth of a sample volume 101 is time-gated by a range gate 119. The gated Doppler signal is supplied to a fast Fourier transformer (FFT) 115 through a sample hold circuit 111, and a band pass filter 113. The FFT 115 Fourier transforms 128 Doppler signals, which can be obtained by repeating transmission and receiving the signals 128 times for the period of 1/PRF. Thereby, power for each frequency component, frequency spectrum, can be obtained. Such a frequency spectrum is arranged along a time axis as shown in FIG. 2, and displayed on a monitor 117 with brightness in accordance with power. Since such an image is often called a spectrum Doppler image, the name spectrum Doppler image is used hereinafter.
An observer can obtain various data from the spectrum Doppler image. There are indexes such as an RI (Resistance Index), a PI (Pulsatility Index) other than information directly obtained from the Doppler image. For example, RI can be obtained by dividing a difference between a maximum velocity (maximum frequency) and a minimum velocity (minimum frequency) in one cardiac cycle by the maximum velocity. PI can be obtained by dividing a difference between the maximum velocity and an average velocity (average frequency) in one cardiac cycle by the average velocity. Most of indexes can be calculated by substituting a characteristic value extracted from the spectrum Doppler image for a predetermined equation.
However, the operation for calculating the indexes has the following problems:
(1) Since an operator must extract the characteristic value from the spectrum every time phase, a long processing time is required.
(2) The spectrum is lifted up by a contrast enhance effect of contrast enhance agent as shown in FIG. 3. As a result, the maximum frequency is shifted, and the value of the index is changed.
(3) As shown in FIG. 4, there is a case in which a sample volume is detached from an interested blood vessel by influence of a motion such as a breath motion, a pulsation, etc. In this case, the index is regarded as an error.